Progress 08/15/03 to 08/14/05

Outputs
The aquaculture industry has considerable evidence that salmon and trout are relatively intolerant of dietary carbohydrate. In this seed grant, we examined the effect of niacin supplementation on carbohydrate and lipid metabolism in rainbow trout (Oncorhynchus mykiss). Niacin (also known as nicotinic acid or vitamin B3) is a potent antilipolytic agent in mammals, reducing lipids levels in the blood. Our working hypotheses were that niacin supplementation would 1) reduce circulating FFA levels in blood, and 2) increase peripheral utilization of glucose and growth when fish receive a high carbohydrate diet. However, unlike mammals, dietary supplementation with niacin (1.0, 1.5, or 3 g/kg) did not lower circulating lipids, enhance carbohydrate utilization, or improve growth performance in rainbow trout. In fact, high doses of nicotinic acid (>1 g/kg) in the presence of elevated carbohydrate (30%) appear to be toxic and may limit feeding, feed conversion, and growth of rainbow trout. At the same time, we also raise new questions about the upper limit of feed levels of carbohydrate in this species. Depending on the source and quality of dietary carbohydrate, the aquaculture industry standard of 20% carbohydrate represents a "conservative" value. We documented outstanding growth performance of trout receiving 24% (mostly wheat flour) or even 30% (mostly purified starch) fed aquaculture rations or to satiety, respectively. Our data also question the recommended levels of niacin (150 mg/kg) in diets of cultured rainbow trout. We saw similar growth performance at very low levels (< 50 mg/kg) of niacin and recommended levels. Measurements of growth performance, plasma substrates, and tissue glycogen showed that elevated dietary carbohydrate (30%) did not affect growth performance, plasma free fatty acids, plasma glucose, and white muscle glycogen in the absence of supplemental niacin, or with the recommended addition of niacin (0.15 g/kg food). However, at higher levels of niacin (1.5 or 3.0 g/kg food), rainbow trout had enlarged livers, and fish receiving the high dose of niacin also on the high (30%) carbohydrate diet gained only 50-60% of the biomass of fish on the low carbohydrate diet. Not surprisingly, apparent retention of energy and protein were also significantly reduced when niacin (1.5 or 3.0 g/kg food) was added to the high carbohydrate diet. Although trout appear to tolerate niacin supplementation up to 3.0 g/kg on a lower (15%) carbohydrate diet, we did not see any positive effects of this vitamin on carbohydrate or lipid metabolism, and caution the use of high levels of supplemental niacin in "high carbohydrate" diets for this species. Despite the negative results, support from 2003-35206-13612 was acknowledged at two national meetings, the Idaho State University Undergraduate Research Symposium, and invited presentations at the Oregon State University and the University of Aarhus (Denmark). A follow-up is now in progress to determine whether the combination of a high carbohydrate and niacin promotes accumulation of triglyceride in the liver of rainbow trout. A manuscript is also in preparation.

Impacts
Our data will lead to a re-evaluation of nutrition guidelines for niacin for rainbow trout and raise new questions about the interaction between blood lipids and carbohydrate utilization in fishes. Our results also question the upper limit of feed levels of carbohydrate in rainbow trout and the use of cellulose as a non-nutritive filler in dietary formulations.

Publications

• No publications reported this period

Progress 10/01/03 to 09/30/04

Outputs
The aquaculture industry has considerable evidence that salmon and trout are relatively intolerant of dietary carbohydrate. In this seed grant, we are examining the effect of niacin on carbohydrate and lipid metabolism in rainbow trout. In mammals, niacin reduces blood lipids and increases carbohydrate oxidation. Our working hypothesis is that dietary supplementation with niacin will reduce FFA levels in trout blood and increase glucose utilization and growth when fish receive a high carbohydrate diet. In the first experiment, 4 groups of fish (n = 150 per group, individual weight 75 g) were stocked into twelve 100-gallon fiberglass tanks (50 fish/tank) with flow-through spring water at 15 degrees C. Fish received one of 4 isocaloric diets: 18% or 24% carbohydrate (with or without 1 g niacin/kg feed). All fish were fed 1% of their body weight daily. Results showed that diet did not affect growth, conversion efficiency, body composition, liver weights, plasma variables (glucose, FFA, triglyceride, and total lipids), or the composition (water content, glycogen and triglyceride) of liver and white muscle. Only blood hemoglobin was significantly lower in fish receiving the lower carbohydrate diet. We attributed this to a non-nutritive filler (cellulose, 88 g/kg feed) and the possibility that cellulose reduced iron absorption. Overall, we were surprised that trout performed well on the 24% carbohydrate diet; however, the conservative feeding level and a high stocking density may explain the lack of any differences between groups. We conclude that, under these experimental conditions, niacin supplementation does not reduce blood lipids or enhance carbohydrate utilization in rainbow trout. In our second experiment, we: 1) stocked fish at a lower density; 2) challenged fish with a wider range of dietary carbohydrate; 3) added multiple levels of niacin; 4) substituted wheat starch for wheat flour; 5) fed fish more of the diet; and 6) performed a 60 versus 90-day trial. Eight groups of fish (n = 81 per group, indivdual weight 120 g) were stocked into 24 tanks and randomly assigned randomly to each of the 8 diets: 15% carbohydrate or 30% carbohydrate with 0, 0.15, 1.5 or 3.0 g niacin/kg feed. Fish are being fed 4 times a day to satiety. Midterm measurements of growth and plasma substrates have been completed. Results show that elevated dietary carbohydrate did not affect growth performance or plasma lipids in the absence of supplemental niacin, or with industry recommended additions of niacin (0.15 g/kg feed). However, at higher levels of niacin, rainbow trout had enlarged livers, and fish also on the high carbohydrate diet gained only 50-60% of the biomass of fish on the low carbohydrate diet. In summary, raising carbohydrate content of the diet to 30% did not affect growth performance, and, although trout appear to tolerate niacin supplementation up to 3.0 g/kg on a low carbohydrate diet, we have not seen any positive effects of this vitamin on carbohydrate or lipid metabolism.

Impacts
This research is addressing whether dietary supplementation with niacin can improve carbohydrate utilization in rainbow trout. If this turns out to be true, implemention of even partial substitution of dietary carbohydrate for fish protein or lipid will benefit the environment and the aquaculture industry. Although the results do not look promising at this time, this research will form the basis for one female graduate student thesis and has provided valuable experience and support for two additional undergraduate students. The negative results will also force us to reconsider the appropriate level and type of carbohydrate to be included in the trout diet, and not assume that metabolic responses are identical in mammals and carnivorous fishes.

Publications

• No publications reported this period